For example, as a conventional variable displacement vane pump applied to a power steering device of a motor vehicle, a vane pump such as the vane pump disclosed in the latter mentioned patent reference 1 has been known.
The variable displacement vane pump of the reference comprises a cam ring that is swingably received in a receiving space formed in a front body, a rotor that is rotatably received in the cam ring and has vanes retractably and projectably received in radially extending slots formed in the rotor, a pressure plate that contacts an inside surface of the rotor, and a rear body that closes an open side of the receiving space of the front body.
The rotor has axially extending back pressure through bores respectively opened to the slots, and the pressure plate has, on an inside surface thereof facing the back pressure through bores, a generally arcuate back pressure groove that is connected a discharge chamber in which a pump discharge pressure is reserved. By introducing the pump discharge pressure to the back pressure bores through the back pressure groove, the vanes are forced to project from the corresponding slots and contact to an inner cylindrical surface of the cam ring, so that pump chambers are formed each being defined by mutually facing adjacent two vanes, an outer cylindrical surface of the rotor, the inner cylindrical surface of the cam ring, an outside surface of the pressure plate and an inside surface of the rear body.
The pressure plate and the rotor have, at respectively contacting surfaces thereof, radially spaced several annular zones each having a plurality of dimples depressed, each dimple having a generally arc-shaped cross section. The dimples function to lubricate the respective contact surfaces of the pressure plate and the rotor by temporarily reserving a high pressure oil that has entered thereinto from the back pressure groove of the pressure plate through a very fine clearance defined between the pressure plate and the rotor. With such dimples, undesired seizing of the mutually contacting surfaces of the pressure plate and the rotor is suppressed.    Patent reference 1: Laid open Japanese Patent Application (Tokkai) 2000-337267